Tesco Bank has been forced to suspend its online transactions after fraudulent criminal activity was spotted on thousands of its customer accounts over the weekend.

A total of 40,000 current accounts were hit by suspicious transactions. Money was pinched from 20,000 of the affected current accounts, Tesco Bank said on Monday morning.

“We apologise for the worry and inconvenience that this has caused for customers, and can only stress that we are taking every step to protect our customers’ accounts,” said the bank’s chief, Benny Higgins.

While online transactions will not be available, current account customers will still be able to use their cards for cash withdrawals, chip and pin payments, and all existing bill payments, and direct debits will continue as normal. We are working hard to resume normal service on current accounts as soon as possible.

Tesco Bank has promised to refund any accounts affected by the fraud and added that it was working with police and regulators to help track down the malefactors behind the crime. The Financial Conduct Authority says it gives “higher priority to the protection of consumers as potential victims of fraud than to the protection of firms themselves as potential victims.” Put another way, banks are expected to act swiftly when such fraudulent activity is detected.

Higgins told the BBC that the bank has around eight million customer accounts. He added that the number of customers hit by fraud was big but not huge. “It’s 20,000 customers, we think it would be relatively small amounts that have come out but we’re still working on that.”

On Sunday, Tesco Bank said that it had “notified some customers that we have blocked their cards to protect their account.”

However, some customers complained on social media about access to their current accounts being frozen without them first being informed of the fraudulent activity.

@tescobankhelp this has left me unable to feed my kids in school tomorrow can’t put money on their dinner accounts can’t buy sandwich stuff